Ok Twinkle,

I'll give it a go as no one else has yet
70 litres equals about 15 (UK) gallons which roughly equals 15" of adult size fish.
You have 3 Platy= 4.5"
4 Swordtails=8"
1 Molly=2"
3 Cory=6" (depending on which type)
All in all 22+" of fish so technically yes, overstocked but, the stock you have now are on the most part hardy so providing you have good filtration and do regular water changes you wont have any problems........however adding female bettas IMO is not a good move
I am no expert (just have a lot to say

) but I believe Molly like a slightly salty water (brackish) but can be ok in fresh water. Female betta are a bit specialised so I'll let someone with a bit more knowledge say why adding them is not a good idea (apart from the overstocking)
